This invention relates to a liquid crystal projector apparatus which includes a liquid crystal display panel for optically modulating light from a light source with an input signal and projects the optically modulated light from the liquid crystal panel to display an image and a driving method for a liquid crystal projector apparatus.
As an example of an image display apparatus, a liquid crystal projector apparatus which makes use of a liquid crystal panel is known. Such liquid crystal projector apparatus include an apparatus called rear projector.
A liquid crystal projector apparatus of the type mentioned uses three liquid crystal panels (also called liquid crystal light valves) for optically modulating the colors of, for example, red, green and blue to combine light of the three components and projects the combined color onto a screen through a lens to display a color image in an enlarged scale. A liquid crystal projector apparatus of the type described includes a lamp as a light source for projecting a video (image) to display it. The lamp generates a great amount of heat and requires cooling thereof.
By the way, liquid crystal panels used in a liquid crystal projector apparatus have such a so-called V-T characteristic (drive voltage-transmissivity) as illustrated in FIG. 18. The axis of ordinate of the V-T characteristic indicates the transmissivity of a liquid crystal panel, and the axis of abscissa indicates the driving voltage (applied voltage) applied to the liquid crystal panel. The V-T characteristic has a characteristic that it is shifted in the direction of the axis of abscissa in response to a variation of the temperature.
The V-T characteristic has a characteristic that, if the temperature rises, for example, from 26.5° C. to 48.6° C. and the V-T characteristic is shifted, then in a gradation portion of the driving voltage of 2.5 V, a drop of the transmissivity of approximately 20%, that is, a drop of the luminance, occurs. Such a variation of the luminance by a temperature variation as just described exhibits its maximum with an intermediate gradation.
Such a drop of the transmissivity of a liquid crystal panel, that is, a drop of the luminance of a liquid crystal panel, as described above is caused by such a shift of the V-T characteristic as shown in FIG. 18 by the temperature of the liquid crystal panel. Therefore, the variation of the luminance of the liquid crystal is not uniform among different gradations. In other words, if a drive signal portion is corrected with a gain or an offset as in correction of the brightness in an ordinary television receiver or the like, then the gradation property of the liquid crystal panel is disordered.
Therefore, for the correction against a temperature variation of a liquid crystal panel, not correction of the brightness but correction of the shift of the V-T characteristic diagram must be performed, and it is known by Japanese Patent No. 2924073 that a shift of a drive voltage (applied voltage) to a liquid crystal panel, that is, a shift of the V-T characteristic, with regard to the axis of abscissa is required.
By the way, where a liquid crystal projector apparatus is used, upon starting of a power supply, the temperature of the liquid crystal panels corresponds to a room temperature. However, after starting of the power supply, since the liquid crystal panels are heated by a light source such as a lamp, the temperature of the liquid crystal panels rises up to approximately 50° C.
The liquid crystal panels are disposed in a housing of the liquid crystal projector apparatus, and the liquid crystal panels are cooled by a cooling fan in the housing. In a liquid crystal projector apparatus of the structure that the liquid crystal panels are cooled by a flow of wind by a cooling fan in this manner, the air in the housing is circulated to cool the liquid crystal panels without taking in external air. The reason why the liquid crystal panels are cooled by air circulation in the housing without taking in external air in this manner is that it is intended to augment the dust-proof performance. In a liquid crystal projector apparatus of such a structure as just described, a fixed time requires until the temperature of the liquid crystal panels rises.
In order to directly measure the temperature of a liquid crystal panel, it is necessary to provide a temperature sensor in a closely contacting relationship with the liquid crystal panel positioned in the dust-proof housing. However, to provide a temperature sensor in a closely contacting relationship with a liquid crystal panel in this manner is difficult due to the following reason in terms of the structure.
As the reason, there is a problem that the temperature sensor cannot be provided at a position at which light of the liquid crystal panel passes, that the area of a portion of each liquid crystal panel at which the temperature sensor is provided is limited because the liquid crystal panels are small in size, that, where the temperature sensor is built in the liquid crystal sensor, an increased cost is required, or the like.
Therefore, it is a possible idea to dispose the temperature sensor at a location in the housing other than the liquid crystal panels, for example, on a circuit board in the housing. Where the temperature sensor is disposed on the circuit board in this manner, a difference appears between temperature rise curves of the actual temperature of the liquid crystal panel and the temperature detected by the temperature sensor on the circuit board.
Where it is intended to indirectly measure the temperature of the liquid crystal panel by means of the temperature sensor provided on the circuit board, since a difference appears between the temperature in the apparatus and the actual temperature of the liquid crystal panel, even if it is tried to measure the actual temperature of the liquid crystal panel, an error occurs. Accordingly, if, when a power supply for the liquid projector apparatus is started, it is tried to indirectly measure the temperature of the liquid crystal panel by means of the temperature sensor on the circuit board and correct the value of the driving voltage to be applied to the liquid crystal panel, then an error occurs.
Usually, in an environment wherein a television receiver in which a Braun tube or the like is used is watched, the room temperature variation during use of the television receiver is, where the room temperature is 25° C., approximately ±10° C. with respect to the temperature of 25° C. However, when a liquid crystal projector apparatus is used, where the room temperature is 25° C., the temperature variation of the liquid crystal panel upon starting of the power supply is a rise of more than 25° C. with respect to 25° C. (50° C.−25° C.=25° C.). If the time of the temperature variation of the liquid crystal panel when the power supply is started is shorter than the time in which the luminance of the light source in the form of a lamp is stabilized, then the temperature of the liquid crystal panel rises in a moment. Accordingly, no particular problem occurs because the user can adjust the picture quality to a stable optimum picture quality while the user does not become aware that the temperature variation of the liquid crystal panel when the power supply is started has an influence on the projected picture quality.
In the liquid crystal projector apparatus, however, since the liquid crystal panels are cooled by circulated air in the housing as described above, the rate of the temperature rise of the liquid crystal panels upon starting of the power supply is lowered by such cooling by the cooling fan when compared with that in an alternative case wherein the liquid crystal panel is not cooled. Consequently, since the rise of the temperature variation of the liquid crystal panel when the power supply is started is slow, a drop of the luminance of the liquid crystal panel occurs, and this gives rise to a problem of at what point of time the picture quality should be adjusted to an optimum picture quality. Since particularly a liquid crystal panel for a liquid crystal projector apparatus wherein air is circulated in the housing as a countermeasure for dust-proof as described above requires a longer period of time until the temperature of the liquid crystal panel rises upon starting of the power supply, a longer period of time is required until the temperature of the liquid crystal panel upon steady operation is stabilized after starting of the power supply, and a variation of the picture quality by a temperature variation of the liquid crystal panel when the power supply is started becomes liable to be visually recognized by the user.